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At least 109 records · Page 6Linked to original sources
USGS1991

Upper Devonian biostratigraphy of Michigan Basin

The Late Devonian Michigan Basin was floored by the Middle and Upper Devonian Squaw Bay Limestone, which was deposited during the downwarping that produced the basin within a former Middle Devonian carbonate platform. The Squaw Bay comprises three beds, each having a different conodont fauna. The two upper beds, deposited during the transitans Zone, have different conodont biofacies that reflect this deepening. The basin was largely filled by the deep-water, anaerobic to dysaerobic, organic-rich, black Antrim Shale, which has a facies relationship with the prodeltaic, greenish gray Ellsworth Shale that prograded into the basin from the west. The Upper Devonian (Frasnian to Famennian) Antrim Shale is divided into four members, from base to top: the Norwood, Paxton, Lachine, and upper members. These members are more or less precisely dated by conodonts. The Norwood was deposited during the transitans Zone to Ancyrognathus triangularis Zone, and the Paxton was deposited from that zone probably through the linguiformis Zone at the end of the Frasnian. The overlying Lachine was deposited during the early Famennian and has yielded faunas of the Upper crepida and Lower rhomboidea Zones. Only the lower part of the upper member is exposed, and near Norwood, Michigan, it yielded conodonts of the Lower marginifera Zone. The widespread Famennian floating plant Protosalvinia (Foerstia) has not yet been found in outcrops of the Antrim, and should not be expected to occur except in the upper member or highest part of the Lachine Member. Its range in terms of conodont zones is from the Upper trachytera Zone through the Lower expansa Zone and possibly into the Middle expansa Zone. One known subsurface occurrence might be datable as rhomboidea or Lower marginifera Zone, depending on gamma ray correlations to outcrops. Black shale deposition ended when the Late Devonian mud delta of the Bedford Shale prograded across the Michigan Basin from the east and then retreated as the regressive Berea Sandstone was being deposited during the major eustatic sea-level fall that ended the Devonian. The Bedford was deposited during the Upper expansa to Lower praesulcata Zones, and the Berea was deposited during the Middle to Upper praesulcata Zones. Both formations contain the spore Retispora lepidophyta, which is a global indicator of latest Devonian age.

Michigan↗
USGS1992

Varied records of early Wisconsinan alpine glaciation in the western United States derived from weathering-rind thicknesses

Weathering-rind thicknesses were measured on volcanic clasts in sequences of glacial deposits in seven mountain ranges in the western United States and in the Puget lowland. Because the rate of rind development decreases with time, ratios of rind thicknesses provide limits on corresponding age ratios. In all areas studied, deposits of late Wisconsinan age are obvious; deposits of late Illinoian age (ca. 140 ka) also seem to be present in each area, although independent evidence for their numerical age is circumstantial. The weathering-rind data indicate that deposits that have intermediate ages between these two are common, and ratios of rind thicknesses suggest an early Wisconsinan age (about 60 to 70 ka) for some of the intermediate deposits. Three of the seven studied alpine areas (McCall, Idaho; Yakima Valley, Washington; and Lassen Peak, California) appear to have early Wisconsinan drift beyond the extent of late Wisconsinan ice. In addition, Mount Rainier and the Puget lowland, Washington, have outwash terraces but no moraines of early Wisconsinan age. The sequences near West Yellowstone, Montana; Truckee, California; and in the southern Olympic Mountains have no recognized moraines or outwash of this age. Many of the areas have deposits that may be of middle Wisconsinan age. Differences in the relative extents of early Wisconsinan alpine glaciers are not expected from the marine oxygen-isotope record and are not explained by any simple trend in climatic variables or proximity to oceanic moisture sources. However, alpine glaciers could have responded more quickly and more variably than continental ice sheets to intense, short-lived climatic events, and they may have been influenced by local climatic or hypsometric effects. The relative sizes of early and late Wisconsinan alpine glaciers could also reflect differences between early and late Wisconsinan continental ice sheets and their regional climatic effects.

California, Colorado, Idaho, Montana, Nevada, Oreg↗
USGS1992

On the age of the penultimate full glaciation of New England

Tills that discontinuously underlie the late Wisconsinan till throughout New England represent the penultimate full glaciation of the region. In southern New England, the late Wisconsinan till and the tills that locally underlie it are informally referred to as upper and lower tills, respectively. For the most part, the ages of the lower tills are not firmly established, and regional correlations between occurrences of lower till, including those on Long Island, New York, are tenuous. Where a lower till underlies deposits having limiting middle Wisconsinan radiocarbon ages (e.g., the Montauk till member of the Manhassett Formation on Long Island at Port Washington, New York, and the lower till at New Sharon, Maine), many workers have assigned the till an early Wisconsinan age. However, lower tills throughout much of New England may be Illinoian or older in age and may correlate with a lower till exposed at Sankaty Head, Nantucket Island, Massachusetts, that is pre-Sangamonian in age. The till at Sankaty Head lies below marine beds containing marine faunas indicative of sea-water temperatures both warmer and slightly cooler than those off Nantucket today and that have uranium-thorium and amino-acid racemization (AAR) age estimates suggesting a Sangamonian age (marine oxygen-isotope stage 5). The lower till at Sankaty Head and the Montauk till member on Long Island were deposited during a full glaciation of New England that was at least as extensive as the late Wisconsinan advance of the Laurentide ice. Global ice-volume data from the marine oxygen-isotope record and the late Pleistocene eustatic sea-level record inferred from raised coral terraces support an advance of this magnitude during marine oxygenisotope stage 6, but not during stage 4. An early Wisconsinan age of the southern New England lower tills and, hence, of the penultimate glaciation there is problematic in terms of the pre-Sangamonian age of the lower till on Nantucket, and in terms of the late Pleistocene global ice-volume and sea-level records. An Illinoian age for the tills and for the penultimate full glaciation of New England is compatible with all the available evidence except some equivocal radiocarbon ages and AAR age estimates.

GSA Special Papers↗
USGS1992

A theoretical model for the flux of radon from rock to ground water

A model is derived to predict the abundance of 222 Rn in ground water in contact with a rock of known uranium content. The model assumes that secular equilibrium is attained in the rock-water system as a whole, but is independent of any microscopic geometric properties of the system. The key variables in the model are bulk properties such as porosity, uranium content of the rock, emanating efficiency, and rock density, all of which are measurable. Thus, the model is simplified by the averaging effects of a macroscopic view of the system. Although less rigorous than other models presented in the literature, it is more generally applicable to natural systems because it does not rely on microscopic properties of the system, which are impossible to quantify. Application of the model to crystalline aquifers in the eastern United States shows that bulk emanation rates of radon are generally less than about 30%.

GSA Special Papers↗
USGS1993

Elk Lake in perspective

Elk Lake is located in the forested region of north-central Minnesota at the headwaters of the Mississippi River and occupies one of countless basins left behind as the last great Pleistocene ice sheet retreated northward into Canada. In this respect it resembles many other moderately deep, dimictic, hard-water lakes in the north-central United States, the sediments of which contain a history of postglacial and Holocene climatic and environmental change. Elk Lake is different, however, because the Holocene sediments in the deeper part of the lake form an uninterrupted sequence of annual laminations or varves. The varves are a chronometer for timing precisely the biologic, geochemical, and sedimentological responses in the lake to cyclic and progressive changes in climate. The varves also, through profound changes in their composition, divide the history of Elk Lake into three, sharply defined episodes; a postglacial lake, a prairie lake, and a modern, mesic-forest lake. We use these episodes and the character of the varves as a framework to guide the reader to the chapters and discussions found in this volume.

Minnesota↗
USGS1993

Geochemistry of surface sediments of Minnesota lakes

Analyses of 36 trace, minor, and major elements were used to classify the sediments of 46 Minnesota lakes. Q-mode factor analyses grouped Minnesota lake sediments according to clastic-, carbonate-, organic-, and redox-related elements. Carbonate lakes occur in west-central Minnesota; their sediments have relatively high concentrations of CaCO 3 , Ba, and Sr. Lakes with sediments containing more than 30% organic matter occur in east-central and northeastern Minnesota; these sediments have high concentrations of organic C, N, and H, and slightly elevated concentrations of Pb. Only three lakes have sediments included in the “redox” group, with relatively high concentrations of Fe, Mn, Mo, La, and Zn. High concentrations of redox-sensitive elements appear to be associated with oxidized iron and manganese minerals, but in Elk Lake, one of the three lakes in this group, a significant amount of iron and manganese is contained in iron phosphate, iron sulfide, and manganese carbonate. Clastic lake sediments are not diluted by large amounts of organic matter, carbonate minerals, or iron-manganese minerals, and are of two types: a western group derived largely from Cretaceous shales in the prairie regions, and a northeastern group derived from Precambrian crystalline rocks in the forested arrowhead region. Western clastic lake sediments have higher concentrations of Al, Na, K, B, Ba, V, Mg, and Sr. Most northeastern clastic lake sediments contain higher concentrations of Cu, Y, Be, and Ni, but several are chemically more similar to those of western prairie lakes.

Minnesota↗
USGS1993

Physical properties, mineralogy, and geochemistry of Holocene varved sediments from Elk Lake, Minnesota

Elk Lake in northwestern Minnesota is situated close to a climatically sensitive ecotone, the forest-prairie border, that migrated back and forth over the drainage basin of the lake during the Holocene. The entire postglacial (Holocene) sediment record in the deepest part of Elk Lake is composed of annual layers (varves) that record the seasonal pulses of many sediment components, and, most important, provide high-resolution (seasonal) time calibration of rates and timing of environmental change. These varved sediments contain many allochthonous and autochthonous components that are sensitive to changing environmental conditions in the drainage basin and the lake. The mineral components of Elk Lake sediments consist mainly of authigenic calcium, magnesium, and manganese carbonate minerals (low-Mg calcite, high-Mg calcite, dolomite, and rhodochrosite), opaline silica (from diatoms), X-ray amorphous iron and manganese oxyhydroxides, and an iron phosphate mineral tentatively identified as rock-bridgeite [(Fe, Mn)Fe 4 (PO 4 ) 3 (OH) 5 ], plus minor contributions from fine-grained detrital quartz, feldspar, illite, and kaolinite. The most notable characteristic of the sediments in Elk Lake is that most of the components were formed in the lake. Q-mode factor analysis of sediment geochemistry reduced 23 observed compositional variables, expressed as percent or parts per million of elements, to three composite variables (factor loadings) whose “concentrations” are expressed on a scale of −1.0 to 1.0. Factor 1 expresses the composition of the inorganic clastic fraction based on concentrations of Mg, Na, Al, Cr, V, Y, Sc, Ni, Sr, Co, and Cu. Factor 2 expresses the similarities in variations of Fe, Mn, P, organic carbon, and Mo. Factor 3 loadings are a synthesis of concentrations of Mn, S, Ca, La, and Ba. Geochemical characteristics define three distinct chemical stages in the development of Elk lake: (1) a carbonate-, manganese-, iron-, sulfur-rich early-lake stage that lasted from 10,400 to 8200 varve yr; (2) a clastic- and diatom-rich mid-Holocene prairie-lake stage that lasted from 8200 to 4000 varve yr; and (3) a final iron-, manganese-, phosphate-, and organic-rich modern-lake stage that developed over the past 4000 yr. The chemical characteristics of the sediments deposited during these three lake phases can be represented by the average compositions of three groups of samples: (1) the average composition of sediments deposited over the past 2000 yr, representing the modern-lake stage; (2) the composition of a 50 varve sample centered on 5700 varve yr that represents the maximum clastic influx into the lake during the prairie period; and (3) the average composition of sediments deposited over the initial 2000 yr of the lake’s existence (10,400 to 8400 varve yr). The amplitudes of climatic oscillations, as reflected by changes in concentration of many climatically sensitive elements, were greatest during the prairie period, pronounced cycles having periodicities of several hundred years. The extremes of these oscillations occurred within several centuries or less, which suggests that significant changes in climate may occur abruptly and rapidly. Most notably, the end of the mid-Holocene prairie period, marked by a sudden decrease in the influx of detrital clastic material, occurred within a few decades.

Minnesota↗
USGS1993

Stable carbon and oxygen isotope studies of the sediments of Elk Lake, Minnesota

Variations in the ratios of 18 O: 16 O and 13 C: 12 C in calcite throughout the Holocene in Elk Lake, Minnesota, are recorded in three varve-calibrated carbonate cores. Marl in a varved deep-basin (29.6 m) core consists mainly of calcite precipitated from surface waters during the summer and probably provides the least complicated isotope record. Marl in a sublittoral (10 m) core consists of calcite contributed from several inorganic and organic sources and probably is the most complicated of the three isotope records. Calcite from shells of the ostracod Candona ohioensis in the sublittoral core provides a record of shallow-water conditions in Elk Lake for the period between 10,500 and 5500 varve yr. Variations in the 13 C: 12 C ratio of organic carbon deposited in Elk Lake during the Holocene are recorded in organic matter in the deep-basin core. All three oxygen isotope records show that, in general, the 18 O: 16 O ratio in carbonate was enriched in 18 O by several parts per mil during the mid-Holocene relative to the past few thousand years. This pattern of oxygen isotope variation is similar to that observed for carbonate materials from other lakes in the northeastern and north-central United States. Oxygen isotope records from these other lakes also show that the 18 O: 16 O ratio during the early Holocene was lower than during the mid-Holocene, and this pattern has been interpreted as representing a response to a generally warmer and drier climate during the mid-Holocene beginning about 8000 varve yr (the so-called hypsithermal). Ostracod and diatom assemblages from Elk Lake cores show, however, that the lake was colder and more saline than at present until at least 6700 varve yr, with conditions similar to those that exist today in cold prairie lakes of Canada. It may be more appropriate, therefore, to refer to the mid-Holocene in northwestern Minnesota as the “prairie period” rather than the hypsithermal, indicating that the climate was drier, but with no connotation regarding temperature. The oxygen isotope data from the three Elk Lake records for this period are somewhat equivocal. Values of δ 18 O in the marl from the sublittoral core and shells of Candona increase from 10,000 to about 6800 varve yr. However, values of δ 18 O in the marl that accumulated in the deepest part of the lake over the same interval (10,000–6800 varve yr) are more or less constant and enriched in 18 O; this probably reflects the cold, saline prairie-lake conditions predicted from the ostracod and diatom assemblage data. All three oxygen isotope records show decreases in 18 O: 16 O ratios after about 6800 varve yr in response to an increase in temperature and decrease in salinity of the lake. The 13 C: 12 C ratios in carbonates from all three Elk Lake records show a distinct pattern; the ratio increased gradually from 10,000 to 8000 varve yr going into the mid-Holocene prairie period and then decreased gradually coming out of the prairie period between about 5500 and 2500 varve yr. These changes in the 13 C: 12 C ratio could have been related to temperature through its effect on solubility of carbon dioxide; however, this interpretation is not supported by the oxygen isotope data. Another possibility is that changes in the 13 C: 12 C ratio are related to organic productivity that removes 13 C-depleted organic carbon and results in 13 C-enriched surface waters. This interpretation implies that organic productivity was higher in Elk Lake during the mid-Holocene prairie period. Support for the high-productivity, 13 C-enriched surface-water model for the mid-Holocene prairie period in Elk Lake is provided by changes in the 13 C: 12 C ratio of organic carbon in the deep-basin core. These changes parallel almost exactly those in the 13 C: 12 C ratio of carbonate carbon, but are about 2% larger (about 6% as opposed to about 4% for carbonate carbon). The difference of about 2% may represent 13 C depletion due to CO 2 limitation. The percentage of organic carbon in the sediment did not increase during the prairie period because it was diluted by an increased flux of detrital clastic material. The ultimate burial rate of organic carbon increased considerably, however, indicating that organic productivity was higher and/or the degree of preservation increased. Diatom assemblages and plant-pigment concentrations indicate that productivity was higher during the prairie period. Pyrolysis hydrogen and oxygen indices show that the 13 C-enriched organic matter that accumulated during the prairie period was hydrogen rich and oxygen poor relative to organic matter that accumulated before and after. These two indices demonstrate that the organic matter that accumulated during the prairie period was much better preserved.

Minnestoa↗
USGS1993

Climatic and limnologic setting of Elk Lake

Elk Lake is located on the Itasca moraine near the source of the Mississippi River in northwestern Minnesota. The basin is in calcareous glacial drift, and the lake water is a dilute solution of calcium and magnesium bicarbonate. Low-magnesian calcite formed by precipitation from the lake water has been a major component of the sediment throughout the lake’s history. The sediment also is laminated with alternating light and dark, millimeter-thick layers containing diatoms, organic matter, Fe(OH) 3 , and CaCO 3 . The sediment microstratigraphy has been preserved because the lake is unusually deep (maximum depth is 30 m) for its size (surface area is 1.01 km 2 ). Oxygen is present in low concentrations or absent in the deepest water during summer and winter. Water movements in the deepest part of the lake are insufficient some years for the complete aeration of the deepst water during spring and autumn circulation periods. Phytoplankton photosynthesis, which occurs mostly in the surficial 6 m of water, typically removes 0.5 g C m −2 day −1 from the epilimnion, which becomes strongly oversaturated with calcite during late spring and summer as the pH increases above the equilibrium pH (7.73) for calcite saturation. Most of the CaCO 3 that makes up the light-colored layers of sediments probably is formed during the late summer, when concentrations of calcium in the epilimnion decrease most rapidly. The silica and organic matter that form the darker sediment laminae are deposited earlier in the year, during a period extending from April to late June, when silica decreases fastest in the epilimnion.

Minnesota↗
USGS1993

Holocene climatic and limnologic history of the north-central United States as recorded in the varved sediments of Elk Lake, Minnesota: A synthesis

Integration of the results and interpretations of geochemical, paleoecological, and sedimentological analyses of a varved sediment record provides a detailed chronicle of limnological and climatic changes for the past 10 ka at Elk Lake, west-central Minnesota. The early Holocene record at Elk Lake was controlled by circumstances of glacial history (e.g., basin morphometry and surrounding till lithology) in combination with global warming at the end of the Pleistocene. Later, the interplay of climate change and a disintegrating ice sheet determined the character of local environments that were affected by reduction of precipitation and increased windiness during the middle Holocene. Elk Lake became more productive and clastic sediment increased to dominate the record as the forests thinned and prairie vegetation characterized the region. During this prairie period, winters may have been cold because disintegrating northern ice sheets ceased to block winter outbreaks of Arctic air. Correlations of wind-deposited materials throughout much of the eastern two-thirds of the United States suggest that drought conditions and strong winds were widespread between 8 and 4 ka. The mid-Holocene was climatically variable, however, with strong fluctuations in varve thickness at decadal, centennial, and millennial scales testifying to rapid climatic changes. Although the cause of such climatic cycles is not yet clear, correlations between 14 C anomalies and varve thickness suggest that variations in solar flux and resulting magnetic storms and zonal winds may have induced strong climatic changes. A particularly strong 600 yr fluctuation to cool and wet climates that may document neoglacial conditions around 5 ka interrupted the prairie period. After 4 ka, the climate at Elk Lake was dominated by a tropical airstream during the summer, and dry arctic and Pacific airstreams during the winter. Large-scale variations ceased, although decadal and multi-decadal variations in varve thickness chronicle changes similar, but not clearly correlative, to historically documented climatic episodes such as the Medieval Warm Period and the Little Ice Age.

Minnesota↗
USGS1993

Chronology of Elk Lake sediments: Coring, sampling, and time-series construction

A 22 m series of cores from a continuously laminated sequence of postglacial sediment was recovered from 29.6 m of water from the deepest part of Elk Lake, Clearwater County, Minnesota, by piston and freeze-coring methods during the winters of 1978 and 1982. A varve time series constructed and used as a basis for subsampling the cores and samples, based on the varve chronology, allows precise determination of fluxes of geochemical and biological sediment components. Chronological and petrographic studies have shown that the laminations are varves and their measurement and enumeration has produced a 10,400 year time series that estimates the rates and timing of paleolimnologic and paleoenvironmental changes in Elk Lake and its drainage. A radiocarbon date from surface sediment is 850 years. The difference between radiocarbon and varve dates continues down core; varve dates are older than radiocarbon dates, probably because of systematic incorporation of dead carbon (as bicarbonate) in organic matter in the sediment. Varve-dated boundaries of pollen zones in the Elk Lake cores compare closely with the ages of the same zones in cores from nearby lakes that have been radiocarbon dated.

Minnestoa↗
USGS1993

Modern sedimentation in Elk Lake, Clearwater County, Minnesota

The varved sediments of Elk Lake, Clearwater County, Minnesota, contain a 10,000 year record of climatic and limnologic events. Sediment traps deployed in the lake’s water column from 1979 to 1981 and from 1983 to 1984 collected samples that permitted us to identify materials, to see the timing of sedimentation events, and to deduce processes that form the microlaminae within varves. Fall and spring microlaminae consist mainly of sequential accumulations of biogenic silica and resuspended calcific and siliceous materials. Precipitates of iron, manganese, and organic detritus dominate the thin winter microlaminae. Calcific microlaminae are deposited in summer. Concentrated iron and manganese precipitates form when the onset of seasonal circulation (especially in autumn) oxygenates the lower water column, but precipitation of these metals also continues throughout periods of seasonal stratification, when these dissolved elements migrate upward and are converted to particles that rain back to the bottom. Mineraloids dominate the sediment; minerals compose only a minor part and include quartz, calcite, rhodochrosite, and rockbridgeite (iron phosphate). The bulk of the bottom accumulation occurs during the longer, calmer summer and winter periods, but important contributions are also made during spring and autumn overturn events. Sediment resuspended from the shallows accumulates together with newly formed endogenic sediment, and can even briefly dominate the seston in autumn and spring. Vigor and duration of seasonal circulations in the upper water column dictate the amount of resuspended sediment contributed annually to a varve. When abrupt warming within days after ice-out stratifies the lake, sedimentation in that year is diminished by resultant suppression of plankton blooms and lack of vernal resuspension that would normally move sediment from the littoral areas into the deep parts of Elk Lake. Thin sections of varves confirm that resuspension during autumn and spring is a varve-forming process that has probably varied in importance as a function of climate and changing morphometry due to infilling. Through an entire year, sediment traps catch a greater proportion of material from spring and autumn overturns than accumulates on the bottom. Lake morphometry is the most important factor governing sediment resuspension and associated annual accumulation rates in traps.

Minnesota↗
USGS1993

Environment of deposition of CaCO3 in Elk Lake, Minnesota

Elk Lake is near the present forest-prairie border in northwestern Minnesota, and is also located on the boundary between hard-water lakes that are typical of once-glaciated parts of the north-central United States and more saline prairie lakes of western Minnesota and the Dakotas. The sediments of the prairie lakes just west of Elk Lake are unusual in that they commonly contain high-Mg calcite and dolomite in addition to low-Mg calcite, which is the dominant carbonate mineral in most marl lakes. During the mid-Holocene dry period, prairie conditions expanded eastward into the forested regions of Minnesota. Variations in types and abundances of carbonate minerals in the Holocene sediments of Elk Lake recorded this climatic change. Studies of primary productivity, carbonate saturation, water chemistry, and sediment-trap samples show that low-Mg calcite precipitates during the summer, triggered by algal photosynthesis. The epilimnion of Elk Lake is always oversaturated with calcite, and the degree of oversaturation increases progressively during the summer. The pH of the epilimnion increases from <8.0 after spring overturn to almost 9.0 in late summer in response to photosynthetic removal of CO 2 during the summer months. The rate of calcium depletion from the epilimnion is proportional to the increase in pH and the rate of photosynthetic carbon fixation. Today the only carbonate minerals that are accumulating in the sediments of Elk Lake are low-Mg calcite and manganese carbonate (rhodochrosite). Rhodochrosite, and probably manganese oxyhydroxide, precipitates when manganese-rich anoxic bottom waters come in contact with carbonate-rich oxic surface waters. During the arid mid-Holocene prairie period, however, low-Mg calcite, dolomite, aragonite, and rhodochrosite all accumulated in the sediments of Elk Lake. Dolomite formed in Elk Lake during this period in response to a higher Mg:Ca ratio in the water, just as it is forming today in lakes of the prairie regions of western Minnesota. The coincident occurrence of aragonite and biological indicators of high salinity suggests that the salinity of Elk Lake and the Mg:Ca ratio were higher than in any of the present prairie lakes of western Minnesota.

Minnesota↗
USGS1993

Entrenchment and widening of the upper San Pedro River, Arizona

The San Pedro River of southeast Arizona is a north-flowing tributary of the Gila River. The area of the drainage basin upstream of the 40-km-long study reach is about 3,200 km 2 . This study traces the historical evolution of the San Pedro River channel—specifically, the deepening, widening, and sediment deposition that have occurred since 1900—and it aims to evaluate the causes of channel widening and deepening, the rate of widening, and the present stability of the channel. Alluvium of the river valley consists of upper Holocene pre- and postentrenchment deposits. The pre-entrenchment alluvium, which forms the principal terrace of the inner valley, accumulated between about A.D. 1450 and 1900 in a relatively sluggish, low-energy fluvial system with extensive marshy reaches and high water table. In contrast, postentrenchment alluvium, which forms the terrace, floodplain, and channel of the San Pedro River, was deposited in a relatively high-energy, entrenched, and meandering fluvial system. The river flowed in a shallow, narrow channel on the surface of the unentrenched valley before 1890. A series of large floods, perhaps beginning as early as 1881, eventually led to entrenchment of the channel between 1890 and 1908. This deepening placed the channel 1 to 10 m below the former floodplain. The channel has widened substantially since entrenchment through lateral migration and expansion of entrenched meanders; its present size is 5.7 times greater than before entrenchment. The rate of channel expansion, however, has decreased since about 1955, coincident with a decrease of peak-flood discharge. Channel area increased at 0.1 km 2 yr −1 from entrenchment until 1955; since then the area increased at only 0.02 km 2 yr −1 , suggesting that the channel has stabilized and that further widening will probably be minor under present conditions of land use, discharge, and climate. The reduction of peak-flow rates was related partly to increased channel sinuosity and to development of floodplains and riparian woodlands. The increased sinuosity produced a reservoir effect that attenuated flood waves, and the development of flood-plains enabled flood waters to spread laterally, thereby increasing transmission losses. In addition, flow rates were probably affected by improved land use and changes of rainfall intensity and short-term rainfall patterns, which reduced runoff and decreased the time necessary for channel stabilization. Livestock grazing decreased steadily after the turn of the century, and numerous stock ponds and small water-retention structures were constructed in tributaries. The cumulative effect of these structures probably reduced peak-flow rates. Short-term rainfall patterns of the wet season (June 15–October 15) have probably changed from annual alternation of above- and below-average rainfall to a biennial or longer pattern. Moreover, frequency of low-intensity rainfall (daily rainfall less than about 1.27 cm) was consistently above average for the decade 1957–1967. These factors probably improved conditions for growth and establishment of vegetation both in and outside of the channel. The causes of the large floods that resulted in entrenchment are poorly understood, although climate and land use were key factors. Floods followed closely the rapid settlement of the area brought about by mining activity in the late 1870s; population rose from a few hundred to 6,000 in less than 5 yr. Extensive wood cutting for mine timber and fuel, suppression of wildfire, and reintroduction of large cattle herds undoubtedly exacerbated entrenchment. Flood-producing wet-season rainfall in the Southwest, however, was unusually heavy before, during, and shortly after entrenchment.

Arizona↗
USGS1992

Allochthonous impact-shocked rocks and superimposed deformations at the Beaverhead site in southwest Montana

Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age. The shocked rocks are present in the Cabin thrust plate—one of many in the Late Cretaceous Cordilleran Thrust belt—and hence are allochthonous, having been transported tens of kilometers from the west. They are considered to represent only a piece from the central uplift of an original complex crater at least 75 km in diameter. It is speculated that some of the considerable uplift and erosion inferred to have taken place in Late Proterozoic to early Paleozoic time in east-central Idaho (The Lemhi arch) may be related to the postulated impact event. Furthermore, quasi-circular magnetic and regional gravity anomalies (50 to 75 km diameter) centered south-southeast of Challis, Idaho, may mark the concealed scar of the original impact structure.

Idaho, Montana↗
USGS1994

The Pennsylvanian Fire Clay tonstein of the Appalachian basin—Its distribution, biostratigraphy, and mineralogy

The Middle Pennsylvanian Fire Clay tonstein, mostly kaolinite and minor accessory minerals, is an altered and lithified volcanic ash preserved as a thin, isochronous layer associated with the Fire Clay coal bed. Seven samples of the tonstein, taken along a 300-km traverse of the central Appalachian basin, contain cogenetic phenocrysts and trapped silicate-melt inclusions of a rhyolitic magma. The phenocrysts include beta-form quartz, apatite, zircon, sanidine, pyroxene, amphibole, monazite, garnet, biotite, and various sulfides. An inherited component of the zircons (determined from U-Pb isotope analyses) provides evidence that the source of the Fire Clay ash was Middle Proterozoic (Grenvillian) continental crust inboard of the active North American margin. 40 Ar/ 39 Ar plateau ages of seven sanidine samples from the tonstein have a mean age of 310.9 ± 0.8 Ma, which suggests that it is the product of a single, large-volume, high-silica, rhyolitic eruption possibly associated with one of the Hercynian granitic plutons in the Piedmont. Biostratigraphic analyses correlate the Fire Clay coal bed with a position just below the top of the Trace Creek Member of the Atoka Formation in the North American Midcontinent and near the Westphalian B-C boundary in western Europe.

Kentucky, Tennessee, Virginia, West Virginia↗
USGS1995

Low-grade, M1 metamorphism of the Douglas Island Volcanics, western metamorphic belt near Juneau, Alaska

The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Deformation, metamorphism, and plutonism range from about 120 to 50 Ma. Subgreenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska, record the earliest metamorphic event (M 1 ). The protolith of the M 1 , low-grade metamorphic mineral assemblages is mostly arc-affinity basaltic rocks of the Douglas Island Volcanics. The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low variance assemblages can be attributed to minor differences in pressure ( P ), temperature ( T ), or X CO2 . Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 °C and 2 to 4.8 kbar.

Alaska↗
USGS1995

Jurassic thrusting of Precambrian basement over Paleozoic cover in the Clipper Mountains, southeastern California

The Clipper Mountains in the eastern Mojave Desert expose evidence of Jurassic plutonic intrusion along what was an active thrust at the east fringe of the exposed Cordilleran Jurassic magmatic arc. This event occurred during a period of widespread arc magmatism and intra-arc thrusting in the Cordillera related to subduction under the west edge of North America. Jurassic plutons in the eastern Mojave Desert are compositionally more diverse and more K 2 O-rich than Cretaceous plutons. Late-kinematic intrusion of the Jurassic Goldhammer pluton, exposed in the Clipper Mountains, was along an active ductile thrust fault that put Proterozoic basement gneiss over Paleozoic strata by the time of intrusion. U-Pb geochronology and hornblende geobarometry are interpreted to indicate that the pluton was emplaced at 161 ± 10 Ma at a pressure approximately 0.46 GPa or more. This pressure corresponds to approximately a 17-km depth or more, at least 13 km greater than inferred stratigraphic overburden (2–4 km) at the time of intrusion. The excess we attribute to pre-intrusion tectonic burial from overthrusting of the observed allochthon of Proterozoic basement and (or) from earlier Mesozoic overthrusting. Ductile deformation continued along the observed thrust system during intrusion of the Goldhammer pluton. Fabrics in the pluton and country-rock record ductile shearing that was partly top westward but mostly top-eastward; the shearing began before or during the intrusion and continued during and after intrusion. The Jurassic burial history in the Clipper Mountains parallels that in adjacent ranges to the east, but contrasts with that in ranges to the south and west where exposed Paleozoic rocks were at colder and shallower crustal levels in Jurassic time. The tectonic record in the Clipper Mountains suggests large crustal thickening and topographic uplift that would be expected to leave a sedimentary record in Jurassic basins.

GSA Special Papers↗